Can QSFP28 (aka zQSFP) DACs be used in place of QSFP+ DACs for 40GBase-CR4 Applications?

All:

Have an testbed configuration where we're running 24 servers with Intel XL710DA1 40G NIC into a switch with 40/100G ports. Trying to use DACs to hold the cost down, but parsing the specs for QSFP+ vs. QSFP28 has been frustrating.

It looks like 40GBase-CR4 requires QSFP+ DACs, but the QSFP28 spec seems to imply that it will step-down (auto-negotiate) from 100G to 40G. If that is true, is there any performance penalty in doing so vs. using QSFP+ DACs?

I've been trying to get some real world recommendations in the 40G space, but so far most folks either go 4x10G to 40G or just ramp up to 100G where QSFP28 is required.

Any insights would be greatly appreciated.

Thank you for your time,

Ed Kiefer

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